One type of prior non-volatile semiconductor memory is the flash electrically erasable programmable read-only memory ("flash memory"). The flash memory can be programmed by a user, and once programmed, the flash memory retains its data until erased. After erasure, the flash memory may be programmed with new code or data.
Flash memories differ from conventional electrically erasable programmable read only memory ("EEPROMs") with respect to erasure. Conventional EEPROMs typically use a select transistor for individual byte erase control. Flash memories, on the other hand, typically achieve much higher density with single transistor cells. During one prior art flash memory erase method, a high voltage is supplied to the sources of every memory cell in a memory array simultaneously. This results in a full array erasure.
For one prior flash EEPROM, a logical "one" means that few if any electrons are stored on a floating gate associated with a bit cell. A logical "zero" means that many electrons are stored on the floating gate associated with the bit cell. Erasure of that prior flash memory causes a logical one to be stored in each bit cell. Every cell of that flash memory can only be written from a logical zero to a logical one by erasure of the entire array. Bit cells of the prior flash memory can, however, be individually overwritten from a logical one to a logical zero, given that this entails simply adding electrons to a floating gate the contains the intrinsic number of electrons associated with the erased state.
One prior flash EEPROM is the 28F256 complementary metal oxide semiconductor ("CMOS") flash memory sold by Intel Corporation of Santa Clara, Calif., which is a 256 kilobit flash EEPROM. The 28F256 flash memory includes a command register to manage electrical erasure and reprogramming. Commands are written to the command register from a controlling microprocessor using standard microprocessor write timings. The command register contents serve as input to an internal state machine that controls erase and programming circuitry.
The controlling microprocessor controls the erasure and programming of the flash memory.
One disadvantage of this prior method of controlling erasure and programming using a microprocessor is that it ties up the microprocessor, thus requiring a relatively high level of microprocessor overhead. This, in turn, decreases system throughput.
Another prior flash memory is the 28F001 CMOS flash memory sold by Intel Corporation of Santa Clara, Calif. The 28F001 is a 1 megabit flash memory, which incorporates a prior write state machine. The prior write state machine automatically programs and erases the array upon receipt of a two stage command from the command port. The prior write state machine thus simplifies erasure and programming for flash memory users. The prior write state machine increases system throughput by freeing the controlling microprocessor for other tasks.
A disadvantage of the prior flash memories is that they do not permit repair or replacement of shorted rows within the memory array. Prior flash memories incorporating shorted rows must be discarded.